Once you've 3d printed them, jam a socket-cap M3 bolt into the knob, and a nut into the shuttle. If you heat them up a little, the plastic melts and gives way a bit, resulting in a nice friction fit. You could probably use a bit of epoxy or superglue for extra permanence too.

All fixed!

Mine lacks the satisfying dedent as you turn the knob, but otherwise works a treat. Feels solid and strong.